The WWII blockhouse near Featherwood, a scheduled monument. Best admired from afar on account of it is set into the side of a narrow slippery gully full of nettles and death. 

From the Historic England scheduling page: “The blockhouse, which is now redundant, has been inserted into the narrow valley bottom of the upper Sills Burn where it lies below the sight line. The blockhouse, which lies immediately across the burn from Featherwood Farm, is clearly of military construction and shares similar constructional techniques with other blockhouses on the Range. It is thought that it was constructed by the military and served as an early blast shelter for the protection of the inhabitants of Featherwood during periods of live firing at the Redesdale Range.”

Even from my distant vantage point the impressions in the concrete of the wooden frame were visible. 
I took this photo after recording the Featherwood stone tent, a historic farmhouse modified for military use.